On the F.S. POLARSTERN cruise ANT-IV/3 (6th December, 1985 - 13th March, 1986) multichannel seismic measurements were carried out in parallel with magnetic and gravimetric measurements on 33 lines with a total length of 6,263 km. 3,350 km of the multichannel seismic lines have been processed aboard. The geophysical studies were designed to investigate the structure and geological development of the Weddell Sea continental margin from meridians zero to 60°W, and to define suitable and safe drilling locations for Leg 113 of the Ocean Drilling Program. The main results of the geophysical studies are: (1) The discovery of an approximately N50°E trending failed drift basin, following the trend of a negative magnetic anomaly and a positive gravity anomaly. (2) The discovery of two extensive wedge-shaped and symmetric basement units around a failed drift basin between longitudes 40°W and 20°W. The seismic characteristics, i.e. seismic velocities of 4 km/s and an internally divergent pattern of reflectors suggest that both wedges are formed from extrusive/intrusive volcanic rocks. (3) The confirmation of a major plate tectonic boundary trending approximately N80°E to N60°E, i.e. the EXPLORA-ANDENES escarpment. (4) The confirmation of a glaciogenic progradational wedge beneath the shelf of the Weddell Sea Embayment, made up of several thousand metres of sediments. (5) The definition of 13 suitable and safe drilling locations for ODP-Leg 113.
The preceding results of the Deep Sea Drilling Project/International Program of Ocean Drilling (DSDP/IPOD) have shown that inspection of borehole cores in the Southern Ocean and around Antarctica are necessary for reconstruction of the development of the Atlantic Ocean and for the understanding of the sedimentary deposition which is strongly affected by the palaeo-oceanographic processes. As the geological development of the Weddell Sea plays a key role for the reconstruction of the south-american and african part of the former supercontinent Gondwana, the Federal Institute for Geosciences and Natural Resources (BGR) has decided to conduct a synoptic geophysical survey in the eastern Weddell Sea and in front of the eastern Antarctic continental margin. This survey has been designed as a prearrangement for a larger "presite" program for preparation of future DSDP/IPOD sites. The main focus of the geophysical survey during the first leg of cruise BGR78 from 5th of January to 13th of February 1978 with M.S. EXPLORA has been put on the continental margin of the south-eastern Weddell Sea and eastern Antarctica between 20°E and 30°W and south of 67°S. Especially the lines BGR78-03 to BGR78-23 of seismic reflection measurements have been carried out in this region which were affected by heavy ice conditions at this time. Additionally, on 43 stations sonobuoy refraction measurements have been conducted.